1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices. More particularly, the present invention relates to a method of exposing photoresists using an electron beam exposure apparatus having a variable beam-shaping system and to a method of forming patterns on semiconductor substrates using the same.
2. Description of the Related Art
As semiconductor devices become more highly integrated, the widths of patterns formed on a semiconductor substrate and the gap between adjacent patterns become smaller. Accordingly, exposure techniques for forming patterns on a semiconductor substrate are constantly being evaluated and refined. A conventional one of these exposure techniques is a reduction projection exposure method using ultraviolet light. However, this method is limited in its ability to form patterns having a minute design rule. Recently, though, an exposure technique using electron beams has been widely used for forming ultra-fine patterns on a semiconductor substrate.
One such exposure technique using electron beams is a beam-shaping exposure method. In this method, an electron beam produced by an electron gun is passed through a first rectangular aperture. The first aperture shapes the electron beam such that it too has a rectangular cross section. The rectangular electron beam is then deflected through a second element having an aperture. The second aperture is rectangular but its dimensions can be varied. As a result, the electron beam is shaped so as to have a predetermined cross section. Then, the cross section of the beam is further reduced by means of a reduction lens, whereupon the beam is directed onto a semiconductor substrate coated with a photoresist. In this method also, a desired pattern can formed on the semiconductor substrate by establishing the desired pattern as a plurality of shots, and then exposing the same number of areas of the photoresist as there are shots.
All exposure methods using electron beams, including the variable beam-shaping exposure method, are superior to the conventional reduction projection exposure method in terms of resolution and accuracy. However, recently-developed semiconductor devices have design critical dimensions (CD) of 0.3 xcexcm or less, and exposure methods using electron beams still have problems forming patterns having such small critical dimensions. In particular, as the design critical dimension decreases, the linearity of the variable beam-shaping exposure method deteriorates. Here, the term xe2x80x9clinearityxe2x80x9d refers to the tendency of the difference between a measured (actual) critical dimension and the design critical dimension to remain constant as the design critical dimension changes. That is, if the difference between the measured critical dimension and the design critical dimension remains uniform irrespective of changes in the design critical dimension, the linearity is considered to be good. However, the linearity of a conventional exposure method using an electron beam is unsatisfactory when the design critical dimension is in the range of 0.3 xcexcm.
A first object of the present invention is to solve the above-described problems of the prior art by providing an electron beam exposure method having superior linearity with respect to the forming of patterns having fine design critical dimensions.
Similarly, a second object of the present invention is to provide a pattern forming method having superior linearity with respect to the forming of patterns having fine design critical dimensions.
To achieve the these objects, the present invention provides a method in which the exposure dose is selected based on the critical dimension of the pattern that is selected from among the various patterns that can be formed on the substrate.
In the electron beam exposure method, the design critical dimension (CD) of the selected pattern is determined. Next, the value of the design CD is compared to a predetermined CD value. The exposure process is performed using a first exposure dose of an electron beam, if the value of the design CD value exceeds the predetermined CD value. On the other hand, the exposure process is performed using a second exposure dose of the electron beamxe2x80x94equal to the first exposure dose plus a supplementary exposure dosexe2x80x94if the value of the design CD is less than the predetermined CD value. The predetermined CD is the one at which the linearity of the exposure process would begin to deteriorate if only performed for the patterns using the first exposure dose.
The supplementary exposure dose may have a value of xcex94CORRECT expressed by the following equation:       Δ    ⁢          xe2x80x83        ⁢    CORRECT    =                    design        ⁢                  xe2x80x83                ⁢        CD        ⁢                  xe2x80x83                ⁢        value            -              g        ⁡                  (                      x            CD                    )                            f      ⁡              (                  x          CD                )            
Here, f(xCD) represents the variation of the measured CD in the case of forming patterns having the design CD using various exposure doses, respectively, of an electron beam having a certain acceleration voltage, and g(xCD) represents the measured CD in the case of forming the pattern having the design CD using the first exposure dose of the electron beam. The variable f(xCD) may be a constant or a function of the variation of the measured CD according to changes in exposure dose. The variable g(xCD) may also be a constant or a function.
The supplementary exposure dose may have a value that is inversely proportional to the density of patterns located within a predetermined distance from the selected pattern. In this case, the supplementary exposure dose has a value of xcex94CORRECT*RC expressed by the following equation:       Δ    ⁢          xe2x80x83        ⁢    CORRECT    *          R      c        =                              design          ⁢                      xe2x80x83                    ⁢          CD          ⁢                      xe2x80x83                    ⁢          value                -                  g          ⁡                      (                          x              CD                        )                                      f        ⁡                  (                      x            CD                    )                      xc3x97          R      c      
Here, f(RC) is a weight corresponding to the density of patterns disposed within the predetermined distance from where the selected pattern is to be formed, and 0 less than RCxe2x89xa61.
In a typical application, various ones of the patterns are formed on the semiconductor substrate. The patterns may all be rectangular. In this case, the design CD is determined based on the minimum dimension of the selected pattern. Some of the patterns will have a design CD smaller than the predetermined CD, and so the method will entail forming patterns on the substrate using various doses of the electron beam, respectively. The variations in the dose of the electron beam can be provided by keeping the same acceleration voltage but varying the exposure time.
In the pattern forming method of the present invention, a photoresist film is formed on a substrate. A target area on the photoresist film having a design CD is exposed using a first exposure dose of a shaped electron beam if the value of the design CD is greater than a predetermined value. On the other hand, if the value of the design CD is less than the predetermined value, the target area is exposed using a second exposure dose of a shaped electron beam, the second exposure dose being equal to the first exposure dose plus a supplementary exposure dose. Finally, the exposed photoresist film is developed to form a photoresist pattern.
As was previously described, the supplemental exposure dose may have a value of xcex94CORRECT or xcex94CORRECT*RC.
The substrate may be a transparent substrate. In this case, a phase shift layer and a light shielding layer are formed on the transparent substrate before the photoresist film is formed. Alternatively, the substrate may be a semiconductor wafer.
In the present invention, even when the design CD of a pattern is rather small, the pattern may be formed satisfactorily using an electron beam because the exposure dose by which the pattern is formed includes a supplemental amount designed to overcome those inherent characteristics of the electron beam exposure process which otherwise make forming very fine patterns difficult.